The present invention generally relates to a head space sampler for use with a gas chromatograph and, in particular, relates to a sampler having means for selectively providing either a comparatively higher or a comparatively lower carrier gas pressure.
In a sealed sample vessel, a state of equilibrium exists in the head space above a liquid sample. In the head space the partial pressures of the individual sample components are proportional to their concentrations in the liquid sample. In a sampler operating according to the head space method, a metered volume of sample from the head space of the sample vessel is delivered to the inlet of a gas chromatograph. The composition of the liquid sample in the sample vessel is then determined from the composition of the head space sample.
In one conventional sampler (German Pat. No. 1 284 660), the sample vessels are sealed by a self-sealing diaphragm or septum. A needle is pierced through this self-sealing diaphragm, which needle is connected to the entrance of the separating column of a gas chromatograph. The entrance of the separating column is in turn connected to a carrier gas conduit arranged to be closed by a solenoid valve. When the solenoid valve is opened, the carrier gas pressure at the entrance of the separating column is transferred to the head space of the sample vessel through the needle acting as a capillary whereby an increased pressure is built up therein. The partial pressures of the sample components, however, are not affected. After closing the carrier gas conduit, for example, by means of the solenoid valve, the pressure at the entrance of the separating column breaks down. Now carrier gas plus sample vapor flows from the head space to the inlet portion of the gas chromatograph at the entrance of the separating column. The volume sampled in this fashion is determined by the time interval during which the solenoid valve in the carrier gas conduit is closed.
Ordinarily, to achieve reproducible results and sufficient vapor pressures, the sample vessels are normally thermostated at an elevated temperature (German Offenlegungsschrift No. 2 818 251).
A particular pressure exists for the optimum separation of the sample in the separating column, which pressure should be applied to the entrance of the separating column during the analysis. If there are highly volatile samples to be applied, it may occur that, during the pre-heating in the closed sample vessels, a pressure exceeding the optimum pressure mentioned is created. This occurs most frequently in capillary columns having very small flow resistance requiring very low inlet-pressure for producing the optimum flow.
If such a pressure is chosen after the piercing of the needle into the head space, no carrier gas flows into the sample vessel for building up the pressure, but from the beginning vapor flows out of the head space against the lower carrier gas pressure to the separating column. The flow from the head space to the column is not interrupted thereby, even if the solenoid valve transitorily closed in the carrier gas conduit, is re-opened after the dosing. Therefore, a defined sampling is not possible.